High performance analog-to-digital converters (ADCs) are now widely used in many applications, including RF receivers (e.g., radar) and electronic countermeasures, communication systems, test instrumentation and others, that handle large dynamic ranges of signal amplitudes of high data rate signals. Ideal ADCs have equally spaced levels of voltage references against which the input signal is compared. Ideal ADCs transfer energy from the frequencies of the input signal or signals to other frequencies as a result of the inherent non-linearity of their transfer function. The transferred energy is often referred to as spurs, as they show up as spikes in a spectrogram of the device output when the input is a tone. Most ADCs suffer additional non-linearities. One particular problem in such high performance ADCs is differential non-linearity (DNL) errors. DNL error is generally defined as the difference between an actual transfer function step width of an ADC and the ideal “stair case” where each quantization level is spaced by exactly 1 least significant bit (LSB). Errors are often due to mismatches in the resistance ladder providing threshold reference voltages and its comparator circuits. Spurs can cause a significant degradation for some systems, especially where a large signal is present and the system must reliably detect much smaller signals at the same time. The spurs or distortion can cause false or missed detections. The electronics industry is constantly striving to improve the spurious free dynamic range (SFDR) of ADCs. A receiver with excellent SFDR is able to detect small signals in the presence of much larger ones. Non-linearities, for example DNL errors, effectively decrease a receiver's SFDR rating.
A well-known technique called dithering is often required to maximize SFDR. Dithering is the process of adding an uncorrelated signal, such as pseudo random noise (PRN) or broadband noise, to a desired analog signal prior to the analog input gate of the ADC. A common approach to creating dither is to use a noise or thermal diode whose output is summed with the wanted signal prior to digitization. Although the injected dither does not eliminate the errors, it whitens the resulting errors, spreading the spurs across a wideband of frequencies with much less power at any frequency. Without dither an input signal is repeatedly quantized at a particular portion of the dynamic range with some given DNL errors of the ADC, thereby repetitively providing the same error. The repetition forces the spurious signals to be at a set of frequencies and amplitudes for a given input. Adding dither to the input results in the combined signal being converted at different points in the dynamic range, across a wider set of reference voltages interacting with different ones as the dither varies, even when the wanted inputs signal has a constant waveform. Adding dither improves the resolution and linearity of the conversion by effectively smoothing the quantization errors of the ADC's transfer function. However, while spurs are reduced, a commensurate increase in the noise floor occurs as adding the dither is equivalent to adding noise to the wanted signal. Many conventional systems simply accept degradation of the noise floor to improve SFDR or sub-optimize SFDR to avoid the additional noise.
FIG. 1A illustrates a prior art embodiment of a SFDR maximization, wherein a digital PRN generator 10 generates a random digital signal that is converted to an analog dither signal by a high dynamic range digital-to-analog converter (DAC) 12 coupled to a summer 14, which adds the analog dither signal to an analog input signal before the dithered analog signal is digitized by an ADC 16. The “known” random digital signal is subtracted from the converter response at a digital subtractor 18. This is a more expensive process for dither creation than a simple diode and will not be 100% random.
FIG. 1B shows another common technique for spur reduction, wherein a wideband non-correlated signal is generated using a thermal noise source 20. The signal from the thermal noise source 20 then passes through a low pass filter 22, which either attenuates or passes the signal based on its frequency. The signal is then added to the analog input signal within a summer 14. Depending upon on how much noise must be injected, signal-to-noise ratio (SNR) of an ADC 16 may be unduly sacrificed.